Organic Geochemistry最新文献

Microbial residue carbon (C) serves as a significant source of soil organic carbon (SOC). However, the contribution of microbial residue C to SOC accumulation after the restoration of farmland vegetation remains unclear, making it challenging to comprehend the role of microorganisms in the accumulation and transformation of SOC after vegetation restoration on farmland. Based on field investigations in Guinan County, Qinghai Province, our study focused on the microbial residue C content and its contribution to SOC in farmland, grassland, and shrub forest, employing the analysis of biomarkers. The results indicated that the microbial residue C content increased with the rise of SOC, soil total nitrogen (STN), and enzyme activities. Compared to farmland, grassland displayed significantly lower levels of SOC and STN, leading to reduced microbial residue C content, notably impacting the C content of fungal residues. Despite slightly lower levels of SOC and STN compared to farmland, shrub forest exhibited higher soil microbial residue C content, notably within bacterial residue C, due to elevated soil enzyme activities. The contribution of microbial residue C to SOC decreased as SOC content increased. In farmland boasting higher SOC content, the contribution of microbial residue C to SOC was notably lower compared to shrub forest and grassland with lower SOC content. In conclusion, compared to farmland, both shrub forest and grassland exhibited higher contributions of microbial residue C to SOC, which is favorable for the sequestration of stable SOC.

An intramolecular isotopic study was conducted on natural gases from the Santanghu Basin in China. Propane samples spanned a wide range of Δ_C-T (δ¹³C_central-δ¹³C_terminal) values from –8.4 to 8.5 ‰, δ¹³C_central and δ¹³C_terminal values varied from –37.3 to –25.8 ‰ and –35.7 to –28.9 ‰, respectively. During thermal maturation, the position-specific carbon isotopic composition of propane shifted from δ¹³C_central < δ¹³C_terminal to δ¹³C_central > δ¹³C_terminal, and the δ¹³C_terminal decreased by 6.8 ‰ within a R_o range of 0.46–0.50 %. The intramolecular carbon isotopic rollover and a decrease in δ¹³C_terminal with a slight increase in maturation could be explained by kinetic effects in thermal cracking of kerogen with a low proportion of branched chains. As isomeric groups were rapidly exhausted, propane generation pathway rapidly changed and primary kinetic isotope effect completely shifted from central carbon position to terminal carbon positions in propane. These changes increased carbon isotope fractionation at terminal carbon positions, thereby reducing δ¹³C_terminal values and shifting δ¹³C_central < δ¹³C_terminal to δ¹³C_central > δ¹³C_terminal within a small maturity range. This work demonstrated that position-specific isotope distribution of propane can be applied to elucidate the chemical structure of kerogen.

The Tarim and Junggar basins are major petroliferous sedimentary deposits in China. Some oil reservoirs in these basins experienced superimposed secondary alterations including early biodegradation followed by thermal maturation/alteration (SSA-B&M). Heteroatomic polar organic compounds in crude oil are significantly influenced by SSA-B&M, which play a vital role in understanding the evolutionary history of oil reservoir. However, research on such alterations is limited. The present study employed negative and positive ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to investigate the molecular transformations of heteroatomic organic compounds in crude oils resulting from SSA-B&M. The findings indicate that biodegradation leads to an enrichment of naphthenic acids, while subsequent cyclization, aromatization and thermal cracking reactions enhances the aromaticity of the pyrolysis products of all the oil samples with varying degree of biodegradation. Notably, highest aromaticity was observed for the crude oil that has been most severely altered by biodegradation. During early and moderate stages of biodegradation, the abundances of O2 compounds (molecules contain two oxygen atoms) representing carboxylic acids were continuously increasing. However, at severe biodegradation levels, fatty acids with 1 double bond equivalents (DBE) underwent degradation, while the naphthenic acids were further enriched. Subsequently, during maturation, O2 compounds with 1 to 5 DBE experienced a steady decline in abundance. The naphthenic acids mainly experienced aromatization, while the fatty acids were partly decarboxylized and cracked thereby forming new n-alkanes. Biodegradation led to modification in neutral nitrogen compounds with 3 to 4 condensed rings (DBE = 9–12) and long alkyl side chains, while the impact on basic nitrogen compounds was relatively minimal. After subsequent thermal maturation, both neutral and basic nitrogen compounds with long alkyl side chains underwent cracking, while those with shorter alkyl side chains and higher degrees of condensation tended to remain relatively unchanged. Finally, two new indices were defined and tested to correctly evaluate the maturation and biodegradation level for crude oils that have experienced SSA-B&M.

The dissolved organic matter (DOM) components in lake water have been widely studied; however, few previous studies have considered the growth of Phragmites australis in brackish lakes. It has not been well understood how salinity variations influence the DOM compositions in pore water and its bacterial mechanisms in lakes with Phragmites australis. This experiment included three salinity groups (1,200 mg/L, 3,600 mg/L, and 6,000 mg/L) to study the interactions between bacteria and DOM in pore water under a salinity gradient. The results showed that the maximum fluorescence intensity (F_max) of DOM measured by excitation-emission fluorescence spectroscopy decreased with increasing salinity. Higher salinity reduced the F_max of protein-like substances and resulted in DOM becoming more aromatic. Salinity affected DOM composition due to the responses of functional bacterial communities. Thiobacillus was salt-tolerant and dominated in the sediments, and its relative abundance was negatively correlated with the F_max of protein-like components. The relative abundance of Flavobacterium showed a positive correlation with salinity and a negative correlation with the F_max of the fulvic acid-like component. Pseudomonas, Brevundimonas, and Polaromonas were negatively correlated with salinity and the F_max of the fulvic acid-like component, while being positively correlated with the F_max of tyrosine-like and aromatic protein substances. Higher salinity inhibited the metabolism gene modules of tryptophan and tyrosine. The results of this study may offer a novel perspective on comprehending the biochemical cycling of fluorescent DOM, encompassing tryptophan-like, tyrosine-like, and fulvic acid-like components in brackish lakes with fluctuating salinity.

BrGDGTs, membrane-spanning lipids produced by bacteria, are at the basis of the MBT’_5ME, a biomarker ratio that has been used as a paleotemperature proxy. However, the response of the MBT’_5ME to temperature changes, particularly in freshwater systems, remains incompletely understood. In this study, oxic mesocosms are used to assess the temperature sensitivity of brGDGTs and their producers, sampled from a lake (Lake Rot) and a river (Sihl River) in three different seasons. Three temperature treatments are employed (10, 17.5, and 25 ℃), representing control (in-situ temperatures), cooling, and/or warming treatments, with GDGTs and the bacterial community measured at several timepoints (24 h, 1, 2, 3 and 5 weeks). The control experiments showed that this experimental approach could not replicate natural conditions exactly, with small changes in chemistry (pH, conductivity, alkalinity) and bacterial community composition. Still, our mesocosm setup yielded valuable insights into the temperature-dependent production of lacustrine brGDGTs and MBT’_5ME values, especially in warming treatments, while no response was observed in cooling treatments, potentially indicating limited sensitivity to cold temperatures. In the river mesocosms not the MBT’_5ME but the IR ratio showed a temperature dependency, potentially driven by small changes in the water pH. Coeval changes in the composition of the bacterial community and the MBT’_5ME and IR are determined to constrain potential GDGT producers. Although an increase in MBT’_5ME in response to some warming incubations is observed, the temperature-sensitivity of MBT’_5ME, as expected from GDGT studies on a global scale, is not supported by this experiment.

During the Late Paleozoic Ice Age (LPIA; particularly early Permian), organic material of the alga Tasmanites was extensively deposited in sediments of southern Gondwana, which formed the Tasmanite oil shales that have only been identified in Tasmania, Australia. It remains unclear whether other analogous geological records exist worldwide that formed at the same time, and the origin is enigmatic. This paper reports the first discovery of Tasmanite oil shales in northern Pangea, specifically in the Lucaogou Formation of the Junggar Basin, China, deposited during the Artinskian (ca. 290 Ma). The organic petrological features of Tasmanites are clearly visible, including thick-walled disks with tubes and radially arranged channels. These shales have markedly elevated C₂₈/C₂₉ steranes and tricyclic terpanes/hopanes ratios. Based on inorganic geochemical data, the salinity of the paleo-lake during deposition of the Tasmanite oil shale interval decreased abruptly from saline to brackish–freshwater. In addition, the pH changed from alkaline to nearly neutral, while the redox environment changed from anoxic to suboxic. Enhanced continental chemical weathering and the change in salinity of the paleo-lake occurred at the same time as global warming and large-scale glacier melting during the Artinskian, which promoted the habitat of the low-salinity tolerant Tasmanites. Our results provide new geological evidence for another occurrence of early Permian Tasmanite oil shales, indicating that Tasmanites flourishs and the associated oil shales may have been widely deposited at the end of the LPIA. The flourishment of Tasmanites archives biotic–environmental co-evolution.

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ¹³C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ¹³C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ¹³C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ¹³C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ¹³C values, and the relationships between diethers and methane δ¹³C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ¹³C values of the ANME source methane predicted from those regression lines and the diether δ¹³C values agreed well with methane δ¹³C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ¹³C value, measured in a mixture with phytane and estimated from the correlation of the δ¹³C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ¹³C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.

Aliphatic and aromatic degraded triterpenoids (TTs), including des-A and des-E TTs, were investigated in turbiditic and hemipelagic mudstones from the Miocene Kawabata Formation (Ishikari basin) and Abetsu and Nibutani formations (Hodaka basin) of south-central Hokkaido, Japan. des-A TTs with carbon skeletons of oleanane, ursane, and lupane are derived from angiosperms, while des-E TTs with hopane carbon skeletons are of bacterial origin. These compounds are thought to form through microbial degradation under dysoxic and anoxic conditions. We found that the concentrations and abundances of total degraded TTs, especially des-A TTs, relative to total organic carbon (TOC) were markedly higher in the Kawabata Formation despite significant levels in the Abetsu and Nibutani formations. These results clearly suggested that huge amounts of des-A TTs may have been transported to, and accumulated in, the Ishikari basin during the late Miocene. Degraded TT/TOC ratios are correlated with aquatic macrophyte n-alkane proxy (Paq) values in the Abetsu Formation. Higher Paq values are interpreted as indicating large contributions of aquatic, submerged or floating macrophytes, and are commonly observed in lake and pond environments. Thus, large amounts of degraded TTs were likely produced through the biodegradation of transported angiospermous TTs in dysoxic or anoxic environments, such as ponds and wetlands. Furthermore, we assumed that organic matter deposited in the Hidaka basin was transported from wetlands and marsh areas, especially floodplain lakes, of paleo-Hidaka Island. The class distributions of degraded TTs varied among samples from these formations. The higher relative abundances of des-A lupanes in the Ishikari basin (Kawabata Formation) suggest that TTs were supplied from mountainous forested areas, where lupenoid-producing woody plant taxa may occur. Meanwhile, less abundant des-A lupanes in the Hidaka basin (Abetsu and Nibutani formations) suggest little or no supply of TTs from mountainous forested areas.